Whilst the majority of people in chat rooms are friendly and willing to talk to anyone that joins in with the conversation, there are also a small minority of people with nothing better to do than try to spoil other users' enjoyment of a chat room by being abusive or offensive. However, never attempt to retaliate by being rude back; there are a number of other ways you can deal with these troublemakers without lowering yourself to their level.
However, before you get upset about a message that seems to be offensive - take another look at it - could the poster have actually meant something entirely more innocent? It can be difficult sometimes to extract the real meaning and intonation from plain text messages and what may look sarcastic or rude may have been said in fun. This is why it is important to use emoticons to express the correct tone so it is clear that you are being funny, serious or indeed sarcastic.